This invention relates to the field of elimination of aqueous deposits, particularly water, from pipelines intended for transporting gaseous fluid, as for example, natural gas. The invention relates more particularly to a new method utilizing a vacuum to dry the pipelines and facilitating putting gas into the pipelines without risk from the flammability of the transported gas and formation of explosive mixtures with oxygen.
Cleaning and drying of gas transport pipelines have always posed problems due particularly to the presence of moisture and the later consequences thereof, stemming from the formation of muds, rust and the like. Since the first gas pipelines were put into service, operators have run into difficulties due to plugging phenomenae, which were ascribed originally to the formation of ice created by the intense and localized cooling of the gas. Later the phenomenae were attributed to the presence of gas hydrates (such as CH.sub.4.7H.sub.2 O, C.sub.2 H.sub.6.8H.sub.2 O and C.sub.3 H.sub.8.18H.sub.2 O) able to grow as crystals until these crystals blocked the pipes completely.
A technique now known and used to eliminate and/or to prevent such hydrate plugs consists of injecting into the pipeline an organic product which is absorbent of water and miscible with it, as for example, methanol or ethylene glycol, to constitute plugs which are pushed all along the pipeline to be dried. This known technique, costly and difficult to use, quite often achieves imperfect results. Furthermore, a large amount of dirtied organic products have to be recovered and then separated either by burning or as wastes which of themselves cause pollution.
A better and more effective technique, which has been tested by applicants, consists of performing the drying by vacuum which makes it possible to avoid supplying and consumption of costly materials, as is necessary when purging with pure nitrogen, methanol, cooling water or the like. According to this process, multistage, sliding-vane vacuum pumps, driven by diesel engines, are used to avoid consumption of electricity. After a descent to a vacuum condition which makes it possible to evacuate virtually all the air from a pipeline, the drying is performed under a practically constant absolute pressure until virtually complete elimination of the moisture. Such a technique certainly represents an advance but still is relatively expensive because of the equipment used and high maintenance costs.